The present invention generally relates to a thermal inkjet printhead of improved design, and more particularly to an improved printhead and method for making the same which involves the integration of MOSFET drive transistor circuitry directly within the printhead structure.
A substantial demand exists for printing systems of high efficiency and resolution. To satisfy this demand, thermal inkjet cartridges have been developed which print in a rapid and efficient manner. These cartridges include an ink reservoir in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion thereof from the cartridge. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al.; No. 4,513,298 to Scheu; No. 4,794,409 to Cowger et al.; the Hewlett-Packard Journal. Vol. 36, No. 5 (May 1985); and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference.
In recent years, research has been conducted in order to increase the degree of print resolution and quality of thermal inkjet printing systems. Print resolution necessarily depends on the number of printing resistors formed on the cartridge substrate. Modern circuit fabrication techniques allow the placement of substantial quantities of resistors on a single printhead substrate. However, the number of resistors applied to the substrate is limited by the conductive components used to electrically connect the cartridge to external pulse driver circuitry in the printer unit. Specifically, an increasingly large number of resistors requires a correspondingly large number of interconnection pads, leads, and the like. This causes greater manufacturing/production costs, and increases the probability that defects will occur during the manufacturing process.
In order to solve this problem, thermal inkjet printheads have been developed which incorporate pulse driver circuitry (e.g. metal oxide semiconductor field effect (MOSFET) transistors) directly on the printhead substrate with the resistors. This development is described in U.S. Pat. Nos. 4,719,477 to Hess; 4,532,530 to Hawkins; and 4,947,192 to Hawkins. The incorporation of driver circuitry on the printhead substrate in this manner reduces the number of interconnect components needed to electrically connect the cartridge to the printer unit. This results in an improved degree of production and operating efficiency.
To produce high-efficiency, integrated printing systems as described above, significant research activities have been conducted in order to develop improved MOSFET transistor structures and methods for integrating the same into thermal inkjet printing units. Currently, MOSFET devices are manufactured using a substantial number of conventional masking/etching steps, as described in Appels, J. A. et al., "Local Oxidation of Silicon; New Technological Aspects," Philips Research Reports, Vol. 26, No. 3, pp. 157-165 (June 1971); Kooi, E., et al., "Locos Devices," Philips Research Reports, Vol. 26, No. 3, pp. 166-180 (June 1971); U.S. Pat. No. 4,510,670 to Schwabe; and Elliott, D. J., Integrated Circuit Fabrication Technology, McGraw-Hill Book Company, New York, 1982 (ISBN No. 0-07-019238-3), all of which are incorporated herein by reference. However, it is always desirable in the production of MOSFET devices and thermal inkjet printing systems to reduce the number of necessary materials and manufacturing steps. This results in lower production costs and greater manufacturing efficiency.
The present invention represents an improved MOSFET transistor structure integrated into a thermal inkjet printing system. The integrated thermal inkjet printhead produced in accordance with the invention is highly efficient, economical, and represents an advance in the art of printing technology, as described herein.